Chapter-5|Magnetism and Matter| NCERT 12th Physics:
Magnetic Field Strength at a Point Due to a Bar Magnet:
The Strength of magnetic field at any point is defined as, the force experienced by a hypothetical unit north pole placed at that point. It is a vector quantity. The direction of magnetic field B is the direction along which hypothetical unit north pole would tend to move.
When Point Lies on Axial Line of a Bar Magnet:
Let 2l be the magnetic length of a bar magnet with centre O. The magnetic dipole moment of the magnet be M, where M= m×2l. Let OP= r, is the distance of the point P on the axial line from the centre of the magnet.
Magnetic Strength(force) at P due to South pole of the magnet is